Cermets from molten metal infiltration processing

ABSTRACT

New cermets with improved properties and applications are provided. These new cermets have lower density and/or higher hardness than B4C cermet. By incorporating other new ceramics into B4C powders or as a substitute for B4C, lower densities and/or higher hardness cermets result. The ceramic powders have much finer particle size than those previously used which significantly reduces grain size of the cermet microstructure and improves the cermet properties.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of and claims priority toU.S. patent application Ser. No. 08/829,034 titled “Novel Cermets andMolten Metal Infiltration Method and Process for their Fabrication,”filed Mar. 31, 1997, now abandoned, incorporated herein by reference.This application is a Continuation-in-part of and claims priority toU.S. patent application Ser. No. 10/260,121 titled “Process forFabrication of Cermets,” filed Sep. 27, 2002, now U.S. Pat. No.7,879,285, incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the United States Department ofEnergy and Lawrence Livermore National Security, LLC.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to processing of cermet materials, andmore specifically, it relates to the fabrication of cermet materials bymolten metal infiltration of metals into ceramic bodies.

2. Description of Related Art

Ceramic materials have been used in advanced body armor systems toreduce weight and still defeat armor penetrating projectles. Suchprojectiles can be defeated if the point of the projectiles can beblunted by the armor when a harder material (like ceramics and/orcermets) is used at the impact point. Body armor has required one of thelightest weight ceramic, boron carbide, with a fiber polymer compositebacking to meet past requirements for the best body armor. New threatsand the need for lighter loads for solders to carry into battle havedriven new requirements for improved materials in body armor and vehiclearmor. Current ceramic and cermet armors can not achieve this newrequirement.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide cermet materialsthat are processed by molten metal infiltration of ceramics.

It is another object of the present invention to provide cermetmaterials that have properties that are useful in armor applications.

Still another object is to provide a process that is cost effective toform molten metal infiltrated cermets with properties useful inapplications like armor, cutting tools, wear parts, etc.

These and other objects will be apparent based on the disclosure herein.

Prior to the parent application, lightweight cermets fabricated by themolten metal infiltration process were limited to boron carbide (B4C)infiltrated with aluminum alloy. See U.S. Pat. No. 4,718,941. Thepresent invention utilizes developments in the areas of lightweightceramics, lightweight alloys, and finer starting ceramic powders toprovide new cermets with improved properties and applications. These newcermets have lower density and/or higher hardness than the B4C cermet.By incorporating other new ceramics into the B4C powders or as asubstitute for the B4C, lower densities and/or higher hardness areachieved in these new cermets. The ceramic powders have much finerparticle size (nano-powders, 40 μm to 900 μm) than those previously used(B4C powders) which significantly reduces grain size of the cermetmicrostructure and improves the cermet properties over that of priorcermets (B4C—Al). These improved cermets also use lower density alloysto improve these properties and to reduce the overall cermet density.

The B4C—Al cermet was developed for opaque armor applications, bothmilitary and civilian. The present cermets can replace the B4C—Alcermets in these applications and, due to the improved properties, couldfind new applications in industry and in the military complex. The newapplications include electrical switches, medicial instruments, wearparts and cutting tools.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a partof the disclosure, illustrate embodiments of the invention and, togetherwith the description, serve to explain the principles of the invention.

FIG. 1A shows a metal alloy on a ceramic sponge prior to metalinfiltration.

FIG. 1B shows the molten alloy infiltrating the ceramic sponge.

FIG. 1C shows an infiltrated cermet body after complete infiltration ofmolten alloy into the ceramic sponge.

DETAILED DESCRIPTION OF THE INVENTION

Recent development of a wide variety of nano-powders has resulted in theimprovement of many products and development of other products for newapplications. Similar improvements are gained by applying thisnano-powder technology to cermets. This is especially beneficial in themolten metal processing of cermets since lower temperatures and shortertimes at temperature allow consolidation of the present cermets. Theseprocessing advantages prevent coarsing of the microstructure duringprocessing and provide a finer grain structure in the cermets, whichincreases performance over previous cermets. This finer microstructureis enhanced further by the additions of lighter weight and/or harderceramic powders in the nano-powder state.

The partial or complete substitution of boron carbide powder with theseceramic nano-powders before molten metal infiltration gives lighterweight and/or harder cermets, which results in a direct improvement inballistic performance for armor applications. These new ceramicadditions include borides, nitrides, and/or carbides of the lighterelements, e.g., Al, B, C, Mg, Be, Ca, Si, Li, Ti and V. These newceramic substitutions also allow the additional options of using lighterweight alloys that wet the compacted ceramic bodies used in the moltenmetal infiltration process. A wider variety of alloys can now be usedfor molten metal infiltration due to improved wetting between theseblended ceramics and the molten alloys. These alloys include, but arenot limited to, combinations of the light elements of Al, Mg, Be, Li,Ti, V, etc. The general properties of desired alloys usable in thepresent invention are low density alloys (less than 2.8 gm/cc) that wetthe ceramics at a reasonable temperature (<1150 C) during a relativelyshort infiltration period (<3 hours).

The new ceramic powders can be formed from the elements or from thepre-reacted compounds of borides, nitrides and/or carbides. Blending ofthe compounds is preferred in most cases due the reactive nature and/ortoxicity of some elements. Blends of these nano-powder ceramics can beprepared by several methods (flame spray, milling, and/or sol-gelprocesses). Nano-powders of B4C can be mixed with various amounts ofother nano-powders to make up the final powder blend needed to form thepowder compact for molten metal infiltration. The powders should bepressed to greater than 55% of theoretical density for armorapplications. Other applications may require less ceramic and more metalcontent for higher tensile strength and toughness but less hardness.

The metal alloys used in the molten metal infiltration process areselected for their wetability with the specific ceramic composite at alow infiltration temperature (<1150 C). The better the wetting of themolten alloy to the cermaic, the shorter the time at temperature fornearly complete infiltration (>98%). Similarly, the lower theinfiltration temperature required, the shorter the time needed tocomplete the infiltration. Less time at the infiltration temperatureresults in a finer microstructure and better performance of the cermet.A typical example of forming these new cermets is the selection of thenano-powder blend of boron carbide aluminum boride and magnesium boride.This powder blend would be cold pressed to greater than 55% density andheat treated in a protective environment (vacuum, argon, etc.) to form aporous component that would have a density lower than B4C if pressedalone. A metal alloy (with good wetting characteristics for thiscomponent) would be melted in contact with the ceramic component at alow temperature in a protective atmosphere (vacuum, argon, etc.) toachieve nearly complete infiltration in a short time (<1 hour).Subsequent heat treatments of the infiltrated component could be made toadjust the properties of the cermet for a specific application. Use ofthe toxic element, Be, is possible if the compositions selected for theceramic compound and/or metal alloy does not exceed toxicity limits setfor this element for the specific application.

The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The embodiments disclosed were meant only to explain theprinciples of the invention and its practical application to therebyenable others skilled in the art to best use the invention in variousembodiments and with various modifications suited to the particular usecontemplated. The scope of the invention is to be defined by thefollowing claims.

1. A method for fabricating a cermet, comprising: providing a ceramicpowder comprising nano-powder size particles; sintering said ceramicpowder at a sintering temperature to produce a porous ceramic preformmatrix; and infiltrating said matrix with a metal alloy to produce acermet, wherein said ceramic powder comprises a blend of boron carbide,aluminum horde and magnesium boride, the method further comprising coldpressing said blend to greater than 55% density, wherein the step ofsintering is carried out in a protective environment.
 2. The method ofclaim 1, wherein said nano-powder size particles comprise a particlesize within a range from about 40 nm to about 900 nm.
 3. The method ofclaim 2, wherein said size of each particle of said particles is aboutthe same as that of all other said particles.
 4. The method of claim 1,wherein said matrix comprises a porosity within a range from about 20%to about 50%.
 5. The method of claim 1, wherein the step of infiltratingsaid matrix comprises heating said metal alloy to at least its meltingtemperature.
 6. The method of claim 5, wherein said melting temperatureis less than said sintering temperature.
 7. The method of claim 1,wherein said alloy is selected from the group consisting of Al, Mg, Be,Li, Ti and V.
 8. The method of claim 1, wherein said alloy comprises adensity of less than 2.8 gm cc.
 9. The method of claim 8, wherein saidalloy is able to wet said matrix at a temperature of less than 1150° C.10. The method of claim 8, wherein said alloy is able to wet said matrixat a temperature of less than 1150° C. within an infiltration period ofless than 3 hours.
 11. The method of claim 1, wherein said metal alloyand said particle sizes are selected in combination so that saidparticles remain of about the same size following the step of sintering.12. The method of claim 1, wherein the step of infiltrating infiltratessaid matrix greater than 98%.
 13. The method of claim 1, furthercomprising heat treating said cermet to adjust the properties of saidcermet for a specific application.